The present invention relates to a tilt angle measuring device which measures a tilt angle of a device main body.
As one of the tilt angle measuring devices, there is a tilt angle measuring device which uses the fact that a free liquid surface maintains a horizontality. Further, as the tilt angle measuring device which uses the free liquid surface, there is, e.g., a tilt sensor.
As regards the tilt sensor, when a device main body, on which the tilt sensor is provided, tilts, the free liquid surface relatively tilts with respect to the device main body.
By entering of a detection light on the free liquid surface, receiving the detection light as reflected on the free liquid surface and detecting a change of a light receiving position caused by the tilt of the liquid surface, a tilt angle of the liquid surface, i.e., the tilt of the device main body can be detected.
In a case where the free liquid surface is used, a followability or a stability of the free liquid surface with respect to a change in tilt of the tilt angle measuring device is affected by a viscosity of a liquid forming the free liquid surface.
That is to say, in a case where the viscosity of the liquid is decreased, the followability is improved, but the stability is lowered, a susceptibility to, a vibration, or the like is increased, and an error incidence rate is also raised. Conversely, in a case where the viscosity of the liquid is increased, the followability is lowered, and an angle detection takes time, but the stability is improved.